Debashis Datta, Senior Manager (Civil & Structural), Institute for Steel Development & Growth, Kolkata.

The retrofitting becomes necessary to enhance the performance of structures including those encountered loss of strength due to deterioration or which have crossed their expected life. The success of retrofitting depends on the actual cause and the measures adopted to prevent its further deterioration. The process involves repair, retrofit, renovation and even part reconstruction.

Steel elements in the form of plates, channels, angles, I-sections along with anchor bolts / through bolts could be used with proficiency for retrofitting of a damaged RCC building. This paper has elaborated in detail different aspects of cracks developed in concrete structures with remedial retrofitting measures along with sketches and calculation methods. Some generic examples of retrofitting methodologies applicable to Global and / or Local failures in concrete have been cited in this paper. Along with the conventional measures of retrofitting some special measures / details using steel have also been discussed on case to case basis.

Use of steel profiled sheeting has been demonstrated to replace or retrofit a damaged concrete roof. Steel sections and sheets being lighter their use does not add extra dead load on the existing structure but offers strength to the structural element as well as system.

Detection and Prevention

Any structure needs periodic repairs and regular maintenance to continue its performance of withstanding the existing loads. But due to changing static as well as dynamic behavior of the applied loads, the structures undergo deterioration and ultimately loose material strength to develop cracks. In case of RCC structures, the cause of the failure could be known by studying the crack pattern and its depth, which helps in identification of cause and prescribing measures for repairs/retrofitting to enhance life expectancy of the structure.

The conventional design methodology ensures a life expectancy of an RCC building structure as about 70 years. But studies indicate that RCC structures develop deteriorating signals at half of structure life, demanding repairs / retrofitting / renovation. INSDAG had carried out one such study on the bridges of Indian railways, which establishes this fact by lots of examples all over India. Hence, it is necessary to understand the effective preventive measures against the development of cracks in RCC elements.

Global Symptoms and Local Symptoms

The structural behaviour and its failure mechanisms could be classified broadly as Global and Local based on the symptoms manifested in its failing members. If the redundancy of the structural system as a Global is high, the non-performance of a few members may not lead to the absolute failure of the structure unless redundancy reduces to zero. But the failure of main members may impair the Global system. It is easier to carry out retrofit on the structural elements, if it is confined to the Local one and does not affect the Global system of the structure.

During repair or retrofitting, care needs to be taken towards the release of residual stresses in the existing system by appropriately propping and supporting and the redistribution of the stresses due to the new system. The best method is to apply props and supports to structural members being repaired / retrofitted to get the residual stresses released. Even after the structure gets retrofitted, it is desirable to keep the props and supports in place for some time and remove them sequentially so that the new stress pattern in the structural element can be redistributed evenly without effecting any undue stress concentration at any point of time.

When the symptoms of failure lead the structure to a mechanism i.e. the state of failure, it requires additional props / supports to hold the system during the retrofitting activities. Even sometimes, it is required to renovate a part of the structure as a measure of retrofitting because that improves the health of the structure and increases its life expectancy.

Steel Elements Used in Retrofitting

Steel elements in the form of plates, channels, angles, I-sections along with anchor bolts / through bolts could be used for retrofitting a damaged RCC building. The detailing of the joints of Steel members with the cracked RCC members requires study of the nature of cracks, their extent and depth. Since, it is very easy to give the steel members different shapes, it could be detailed in the most favourable way to fit into the system according to its necessity and available spaces. Some specific failures and its retrofitting measures are explained below.

An RCC beam is prone to fail by either flexure and / or by shear force. The stressed zone for shear in the beam is located near the supports and cracks develop at an angle of 450 starting from the point of support. But the stressed area for the flexure is near the mid-span and its failure crack develops vertically.

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